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IV.—Some Definitions in Dynamical Geology
Published online by Cambridge University Press: 01 May 2009
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In view of the active discussion of the problems of earth-movement and mountain-growth now current, certain fundamental definitions, growing out of the discrimination of processes commonly confounded but really distinct, seem to be timely.
The various processes with which the geologist has to deal fall naturally into two principal and antagonistic categories and five subordinate and supplemental categories; and each category, great and small, comprises two antagonistic classes of movements or agencies.
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page 493 note 1 A term proposed by Gilbert.
page 494 note 1 Singularly, Reade and all his predecessors neglected an important (in the judgment of the writer the most important) factor in the movements contemplated in the hypothesis. Lines of sedimentation are the margins of continents, and the sediments are laid down not upon horizontal surfaces, but upon seawardly sloping bottoms; so the sediments do not form horizontal beds, but take a variable seaward slope determined by marine currents, wave action, etc. Thus the mass of sediments is collectively in the condition of a mass of snow upon a roof or upon a mountain side, i.e. in a condition of potential instability or inequipotentiality. If the mass is stable in either case, it is because the friction among the particles exceeds the attraction of gravitation upon the particles; it is obvious that if particle friction were reduced by augmentation of temperature or by alteration of constitution, or if the efficiency of gravitation were increased by addition to the mass, the point of stability might be passed, when the mass would move in the direction of the slope; and it is equally obvious that if an inequipotential mass expand, the resulting movement will not take place equally in all directions, but mainly or wholly in the direction of least resistance, which is that of the slope. Since the sediments fringing continents are in a condition of inequipotentiality, any movement due to the rise of isogeotherms or other cause must take place in a single direction; and it might not be limited to that due to expansion, for other factors co-operate. Supplemented by this additional conception, the hypothesis of mountain growth so ably advocated by Herschel, Babbage, Hall, Dana, Le Conte, Reade, and a score of others, appears to gain much in acceptability.